With the recent reduction in size, weight, and thickness of electronic equipment, a high-density printed circuit board with a very high multilayer configuration has been developed.
On the other hand, a conventional multilayer printed circuit board is made by laminating a plurality of printed boards 1 as shown in FIG. 1, and conventionally, an interlayer electrical connection is formed by plating through holes 2 obtained by penetrating the multilayer printed circuit board. In FIG. 1, reference No. 3 denotes a printed wiring. It is necessary to plate these through holes of the multilayer board, and to plate holes serving as inner via holes between adjacent two layers. This inner via hole 7 is, for example, a hole installed on a substrate 5 of a single printed board 4 constituting the multilayer board, as shown in FIG. 2. In the case of this printed board 4, a copper foil 6 constituting a wiring pattern is fixed on both surfaces.
However, the production process is complicated because plating both the through hole 2 and an inner via hole 7 involve a wet process. Furthermore, when a through hole is formed, considerable limitations are placed on parts packaging of the printed circuit board, and it becomes impossible to perform high-density parts packaging.
Therefore, a method of forming an interlayer electrical connection by using a conductive paste has recently been suggested and put into practice. According to this method, it is possible to form an electrical connection between the respective layers at a predetermined position, thereby markedly improving packaging density.
The conductive paste used in a conventional printed circuit board is known as a conductive paste for a jumper circuit, a conductive paste for EMI (electromagnetic interference) shielding, or a conductive paste for a through hole. However, these conductive pastes have not been used for via-filling because they have not been specially designed for via-filling.
The properties required for a via-filling conductive paste composition for a printed circuit board are as follows:
(1) Increased contact probability between metal particles to reduce via connection resistance. PA1 (2) Proper viscosity characteristics to allow screen printing. PA1 (3) A conductive paste composition that firmly bonds to a copper foil. PA1 (4) Absence of voids or cracks in the conductive paste filled in the vias. PA1 (5) Minimal migration between adjacent vias. PA1 (6) A sufficient useful life. PA1 (7) Sufficient reliability (e.g., endures soldering shock test and heat cycle test).
Japanese Unexamined Patent Application Publication No. 7-176846 (hereinafter referred to as a "prior publication 1") discloses a via hole-filling conductive paste composition comprising 80-92% by weight of a conductor filler having an average particle diameter of 0.5 to 20 .mu.m and a specific surface area of 0.1 to 1.5 m.sup.2 /g, 8-20% by weight of an epoxy resin having two or more epoxy resins, which has a cold viscosity of not more than 15 Pa.multidot.s, and 0.5-5% by weight of a curing agent. Although prior publication 1 discloses an epoxy resin as the resin component, the epoxy resin alone cannot inhibit an increase in electric resistance when the conductor has been oxidized.
Japanese Unexamined Patent Application Publication No. 56-8892 (hereinafter referred to as a "prior publication 2") discloses composite metal powders made by coating a copper powder with silver as a conductive paste for printed circuit board. However, the invention disclosed in prior publication 2 is used for the purpose of preventing a soft solder from transferring to the silver during soldering, not for the purpose of improving the reliability of an electrical connection between copper particles. Therefore, prior publication 2 does not disclose that the epoxy resin and resol-type phenol resin are used as a binder.
In addition, using copper particles as a conductor provides a benefit because copper is economically advantageous compared to gold, silver, and palladium as a noble metal, and is not as greatly influenced as these noble metals by a change in price resulting from a variation in demand. Copper also has a beneficial feature in that the electric resistance of copper is lower than that of nickel, tin, and lead. On the other hand, copper has a disadvantage in that it is inferior in operation reliability because it is oxidized more readily than gold, silver, palladium, nickel, tin, and lead. This is a particular disadvantage because a via-connected copper paste is gradually oxidized in an environment of high temperature and high humidity, and the electric resistance increases resulting in a broken printed wiring in a severe case.
As described above, a via-filling conductive paste composition having excellent characteristics has not yet been provided.
Under these circumstances, the present invention has been accomplished. In order to improve the reliability of an electrical connection between copper particles used as the conductor component, sufficient tests and technical studies have been performed to obtain proper properties for use as a via-filling conductive paste with respect to the proportion of the copper particles and the respective blending components. As a result, the present invention has been achieved. An object of the present invention, therefore, is to provide a conductive paste composition, which is superior in electrical connection reliability and has the above properties required of a via-filling conductive paste composition.